This relates generally to imaging systems, and more particularly, to imaging systems with tungsten light shields and in-pixel light grids.
Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors. Imaging systems (i.e., image sensors) often include a two-dimensional array of image sensing pixels. Each pixel typically includes a photosensitive element such as a photodiode that receives incident photons (light) and converts the photons into electrical signals. The imaging system contains an image sensor die having an array of photodiodes formed in a semiconductor substrate. The image sensor die is mounted on a digital signal processor (DSP) die.
In particular, the image sensor die includes active photodiodes that receive incoming light and reference photodiodes that do not receive incoming light. A metallic light shield such as a tungsten light shield is formed over the reference photodiodes to prevent the reference photodiodes from receiving light. In one conventional arrangement, the tungsten light shield is shorted to ground by forming a via from the tungsten light shield to a deep contact in the semiconductor substrate. The formation of the via from the tungsten light shield to the deep contact, however, requires extra processing steps and thereby increases cost. Contact to the silicon substrate relies on adequate doping of p-wells or n-wells in the silicon substrate to form an Ohmic contact. Otherwise, there will be high contact resistance, and the tungsten shield will not be properly grounded. For deep p-wells or n-wells, the doping at the bottom may not be adequate to form a robust Ohmic contact. In another conventional arrangement, the tungsten light shield can be shorted to ground by coupling the tungsten light shield to the aluminum wire bond pad via an aluminum strap. The formation of the aluminum strap coupling the tungsten light shield to the aluminum wire bond pad, however, yields an undesirable topography.
In-pixel light grids have also been developed to help improve isolation between the active photodiodes in the array. The in-pixel light grids, however, are not shorted to ground, which can be problematic and can cause undesired coupling effects.
It would therefore be desirable to provide improved ways of biasing the tungsten light shields and the in-pixel light grids.